(1) Field of the Invention
The present invention relates to a semiconductor laser light modulation and demodulation system in which the semiconductor laser light is modulated and demodulated for realizing a light communication system. More particularly, the present invention is to stably modulate and demodulate the frequency of semiconductor laser light which does not have a narrow spectrum band width and hence the oscillating frequency is usually unstable.
(2) Prior Art
A semiconductor laser light device has hitherto been used conveniently in view of its compact size and easy handling. This kind of semiconductor laser device is applied for instance in a light communication system. Such a communication system is sub-classified into two systems. One of which is incoherent system, in which the strength or intensity of the laser light is modulated. The other one is coherent system, in which the phase or frequency of the laser light is modulated. In so far, only the incoherent system of modulating the strength of the laser light has been used. As a principle of the laser light strength modulation, a modulation signal is superposed on the driving current supplied to the semiconductor laser element and the variation of the laser light strength at the oscillator output varying according to the modulation signal is utilized.
However, in such an incoherent strength modulation of the laser light, the variation of the strength of the laser light oscillation output inevitably accompanies other variations, like an irregular hopping of the oscillation mode of the laser light due to variation of the driving current. Accordingly, a good signal to noise ratio in the laser light oscillation output has not been obtained. Further with respect to the drift speed of the carrier of the laser light, there was a disadvantage in that the upper limit of the signal frequency being able to be modulated is limited up to some GHz order.
On the other hand, the coherent system, in which the laser light is frequency or phase modulated, is still just in a beginning stage or research stage to check the possibility of realization based on so called "analogie" for the frequency modulation of the electromagnetic wave. Moreover, the semiconductor laser light has a wider spectrum band width and the oscillating frequency itself is unstable. Thus the present problem is how to obtain a narrower frequency spectrum of such a semiconductor laser light and also how to stabilize the oscillation. Under such situation of the development of the technique in this field, a system to realize the frequency modulation or phase modulation by intentionally varying the frequency or phase of the laser light has never been established.